3 STRUCTURE OF GELS 67 



gical colloids it is often difficult to decide upon their nature, I have 

 suggested that they should be called by the neutral name oi points of 

 attachment ot junctions ("Haftpunkte"), which is non-committal as to 

 the kind of bonds involved (Frey-Wyssling, 1933 b; 1936 a). 



In Fig. 5 1 the junctions are marked by black dots. Obviously in a gel 

 with chain molecules many fewer junctions are needed to build up a 

 structure than in the case of spherical colloid particles. A gel built up 

 by high polymer chains can therefore contain up to p/% of water 

 CSeifriz, 1938) and yet possess a structure. This fact is very important 

 to an understanding of protoplasm structure, since the water content 

 of living matter is always surprisingly high. 



In Fig. 5 1 b further chains can be interwoven at will ; the number of 

 junctions will then increase, and the result is a more solid gel structure. 

 The plastic properties of the structure become less pronounced at the 

 same rate, while the elastic properties increase. Thus the model of a 

 gel structure projected here comprises all the states ranging from gels 

 very rich in water to those very poor in water characteristic of active 

 and dormant protoplasm. 



Limited sivelling. In the swelling process the absorbed medium pene- 

 trates into the interstices available in the gel structure and widens the 

 framework. It is clear that the permeating liquid should show chemical 

 affinity for the chain molecules concerned. Thus the lipophilic mole- 

 cules of rubber and polystyrene swell in benzene, which is lipophilic, 

 while the hydrophilic cellulose swells in water. Whether in this process 

 the framework of the gel goes to pieces, i.e., whether the gel is dissolved, 

 depends on whether the junctions present can be disrupted. If the 

 bonds are of the type of cohesive forces and the solvent present is 

 capable of completely solvating the chain molecules, the gel structure 

 may disintegrate and change into a gel solution in which the particles 

 have greater mobility. This happens, for instance, in the swelling of 

 fibre cellulose in cuprammonium. Limited swelling, therefore, always 

 indicates that the chain molecules can only be solvated to a limited 

 extent. 



Sometimes main valency bonds may be among the junctions. For 

 example, as shown by Staudinger (1936a), polystyrene with a degree 

 of polymerization of 1700 is soluble in benzene, but on the addition 

 of traces of divinylbenzene (0.002%) it is converted into a product 

 showing limited swelling in benzene (Fig. 5 2). In the same way chains 



